Disclosed is a Delta-Sigma Analog-to-Digital Converter (ADC) that has a very high sampling rate (over 100 GHz) and which is preferably optically sampled. The sampling rate can be many times higher than the regeneration speed of its individual electronic quantizers by using multiple quantizers in a single Delta-Sigma feedback loop. These multiple quantizers are addressed in a time-interleaved manner, with each quantizer handling only a subset of the sampled input pulses. For an embodiment with N time-interleaved quantizers, the clock speed of the individual quantizers can be 1/N times the sampling rate. Time-interleaved sampled pulses are generated by one or more samplers, which are preferably photonic or optoelectronic samplers, located within the Delta-Sigma loop. The output of Digital-to-Analog Convertors (DACs) in the Delta-Sigma loop are sampled, or gated, preferably by means of optical pulses. The multiple DAC outputs are combined to produce the Delta-Sigma feedback signal.
With this approach, the oversampling ratio can be substantially higher than that which is achievable with prior art Delta-Sigma ADCs. The Delta-Sigma approach disclosed herein derives a superlinear benefit from such oversampling and thus the resolution of this ADC system is increased greatly. With this approach, the analog summing points and the integrators function as continuous-time elements. Since, with continuous-time integrators, the sampling speed is not dependent on the speed of the transistors in the integrators, the ADC can accommodate analog inputs of very high frequencies (over 10 GHz for example). The quantizer and DAC, however, can be considered as discrete-time elements, which is one result of using optoelectronic sampling. This approach also can be applied to high-order Delta-Sigma designs.
Ultra-short and stable optical pulses and photonic or optoelectronic samplers are preferably used to accomplish the sampling. Optical sampling pulses are preferably coded according to their wavelength using a known method. One or more optical wavelength-division multiplexing splitters and associated time delay elements can be used to select and sequence the sampling or sampled pulses for each of the quantizers and gating pulses for each of the DACs.
The response of an electronic ADC can be limited by the regeneration time of its comparators/quantizers. Even for a quantizer with very fast transistors having a fT of 200 GHz, for example, only moderate ADC resolution can be achieved at 10 GSPS rates. Optical sampling is preferred because optical sampling makes use of ultrashort laser pulses with high temporal stability to sample an analog electrical input. Picosecond sampling or aperture windows and sampling-pulse repetition rates of 100 GHz can be achieved with optical sampling. The sampled pulses are time-interleaved among multiple quantizers within a single Delta-Sigma ADC. For example, the Delta-Sigma loop may have ten quantizers, each operating at a clock speed of 10 GSPS. However, the analog difference signal produced by the Delta-Sigma loop is sampled at a rate of 100 GSPS. Thus, the oversampling ratio is determined by the 100 GSPS value. Yet the quantizers only need to operate at rates of 10 GSPS in such an embodiment (with ten quantizers in the loop), and thus the quantizers can be implemented electronically using fast transistors.
This approach to time interleaving is superior to prior time-interleaved approaches that optically sample an analog input and then interleave those samples among independent quantizers. A time-interleaved system based on optical sampling is described in Hamilton and Bell's U.S. Pat. No. 5,010,346. In the present case, all of the quantizers are part of the same Delta-Sigma loop. Thus, they are not independent since their combined outputs provide the difference signal fed back through the Delta-Sigma loop. It is the “delta-sigma” signal that is sampled and time interleaved. This coupling of the Delta-Sigma quantizers achieves greatly improved ADC resolution since a high-order noise filter in the Delta-Sigma loop produces a super-linear improvement in the signal-to-noise ratio.
The disclosed invention makes use of the concepts and components described in the optically sampled analog-to-digital converter (ADC) system disclosed in U.S. Pat. No. 6,781,533 entitled “Optically Sampled Delta-Sigma Modulator” mentioned above. The ADC architecture of this related application incorporates the optoelectronic sampler following the analog integrator and within the loop of a delta-sigma modulator. The noise resulting from the sampler and the spurs generated by the non-linear response of that sampler are thus suppressed by the noise-spectrum shaping and digital filtering. Thus, the resolution (as determined by the signal-to-noise and spur free dynamic range) is improved over what can be achieved with a conventional combination of optical sampling with separate electronic ADC. The ADC architecture disclosed herein can provide additional benefits.
Analog-to-digital converters that are capable of both large bandwidth and high resolution are needed for many applications. Such capability can enhance the capabilities of digital receivers, for example. Such ADCs may even make possible the direct analog to digital conversion of high frequency signals. Thus, one can avoid the complexity and size associated with needing multiple stages of analog frequency conversion prior to the analog to digital conversion in such equipment.
FIG. 1 illustrates the optically sampled Delta-Sigma ADC system disclosed in U.S. Pat. No. 6,781,533 entitled “Optically Sampled Delta-Sigma Modulator” mentioned above. This ADC system includes a delta-sigma modulator loop 100 and a subsequent digital filter 102. A continuous-time analog input signal X(t) is applied to the input node of a delta-sigma modulator loop 100. At the input node, the feedback signal Ya(i) from the loop (an analog representation of the quantized output) is subtracted from the analog input signal at junction 104. The difference signal Xd(t) is then integrated (for a first-order delta-sigma modulator) by integrator 108, the integrator 108 may also be referred to as a feed forward loop filter. The output Xi(t) of the integrator 122 is sampled by the optoelectronic sampler 110. The optoelectronic sampler 110 is controlled by an impulse source 112 to produce a short electrical pulse, and if desired also a short optical pulse, whose amplitudes are determined by the output level (or voltage) of the integrator 108. This short pulse can be optionally broadened by a pulse broadener 114 comprised of, for example, a filter or another integrator. The quantizer 116 compares the peak value of this pulse with a predetermined threshold value and outputs a digital “one” or “zero ” according to that comparison (for a 1 bit quantizer). The digital output stream Y(i) from the quantizer 116 is then processed by the digital filter 102 and, usually, a decimator (not shown). The digital output stream Y(i) at the output node of the deltasigma modulator loop also is directed to a feedback path in which it is converted back to an analog signal by a DAC 118 and, possibly, low-pass filtered by a low pass filter 119 to produce feedback signal Ya(i). The feedback signal Ya(i) is subtracted from a later portion of the analog input signal X(t).
The optoelectronic sampler 110 in both the related patent application referred to above and the present patent application is an element that accepts a first input 120 comprised of a sequence of optical pulses and a second input 122 comprised of an analog electrical waveform (Xi(t) in FIG. 1). The output 124 of the sampler 110 is a sequence of electrical pulses whose amplitudes are determined by the values of the analog waveform at the instances that waveform concurs with the input optical pulses at input 120. The output of the sampler 110 must have at least as many distinguishable levels as needed for the particular design of the feed forward loop. In many cases, only two distinct output levels are needed (that is, a one bit quantizer is suitable in such embodiment). Several possible embodiments of such an optoelectronic sampler 110 are described in the related patent application referenced above and reference may be had to that patent application for the details relating to their construction. Such combined electroabsorption and photodetection devices 110 accommodate very short optical pulses, of even sub-picosecond widths. In addition to the electrical output, the disclosed electroabsorption device 110 also can have an optical output that is a short optical pulse whose amplitude is determined by the analog input voltage. That output pulse likewise can be used as an optical sampled signal that can be photodetected to produce an electrical sampled signal. The electroabsorption device is called, in this patent application, a photonic sampler when its optical output is used or an optoelectronic sampler when its electrical output is used.
In the approach disclosed in the related patent application, the clock speed of the quantizer, and thus the sampling rate of the ADC, is limited by the regeneration time of the comparator circuit in the quantizer 116. Note that the comparator regeneration time is inversely proportional to the fT of the transistors used in the comparator circuit. Even when fast transistor technology is used, such as InP-based HBTs, the comparator-limited sampling rate would be only slightly higher than 10 GSPS.
With optical sampling, the sampling aperture (temporal width) of the ADC is determined by the width of the input optical sampling pulses rather than by the decision time of the quantizer. The optical pulses can have widths of a few picoseconds (10−12 seconds). The sampled pulses can then be stretched in time by means of a pulse-broadening filter 114 so that the frequency content seen by the quantizer can be much lower.
The present invention achieves a net sampling rate that is higher than the quantizer clock rate by time-interleaving the sampled pulses among several parallel comparators and digital-to-analog converters (DAC) located within a delta-sigma loop. This invention also provides a means to combine the multiple DAC outputs at the summing junctions of the delta-sigma loop.
Time interleaving of discrete-time sampled pulses among multiple analog-to-digital converters has been described in prior work. In these prior systems, the time-interleaved quantizers are operated as distinct, uncoupled analog-to-digital converters. In contrast, the present invention couples its time-interleaved quantizers within the same Delta-Sigma loop.
A prior art system based on electronic sampling is described by Schiller and Byrne in IEEE Journal of Solid-State Circuits, vol. 26, no. 12, pp. 1781–1789 (1991). This system is illustrated in FIG. 2a. The analog input signal is sampled by multiple electronic samplers. The time interleaving is achieved by phase shifting the timing clock waveforms that are delivered to the multiple electronic samplers. The output of each sampler is directed to a different analog-to-digital converter (ADC) after that output is broadened by means of a low-pass filter.
Various prior methods to encode and time-interleave the optical pulses have been proposed and demonstrated. According to one method, the optical sampling pulses are encoded so that they have different wavelengths. One example of wavelength-encoded, time-interleaving is illustrated in FIG. 2b and is described by Clark et al. in IEEE Photonics Technology Letters, vol. 11, no. 9, pp. 1168–1170 (1999). According to this approach, spectrally broad pulses from a mode-locked laser are sliced by a wavelength-division multiplexer into N discrete wavelength channels. Each wavelength slice is itself an optical pulse. These wavelength-encoded pulses are then delayed by different time durations. The result is a series of N optical pulses for which each successive pulse has a different wavelength. This series repeats at the repetition rate of the mode-locked laser. In this way, the repetition rate of the optical sampling pulses can be N times greater than the repetition rate of the mode-locked laser. After the optical sampling, the sampled pulses are selected by means of a second wavelength-division multiplexer. Like the other prior art, the wavelength selected, time-interleaved sampled pulses are then distributed among a group of uncoupled analog-to-digital converters.
An electronic delta-sigma analog-to-digital converter with time-interleaved quantizers has been described in U.S. Pat. No. 5,621,408. This approach makes use of electronic sampling that occurs effectively at the multiple quantizers that are located within a single delta-sigma loop. In one embodiment, the outputs from the multiple quantizers are switched so that they sequentially control a single digital-to-analog converter of the loop. In another embodiment, the outputs of the quantizers are directed to multiple digital-to-analog converters. The outputs of those DACs are switched by means of electronic switches. In contrast to this prior art approach, the present invention provides a way to use a single, higher-speed sampler with multiple quantizers of lower-speed. The present invention makes use of optical sampling pulses to achieve much higher sampling rates and much narrower sampling apertures. Also, optically controlled samplers or gates, appropriately delayed with respect to the sampler outputs, are used to activate the DAC outputs at the proper temporal instances.